1. Field of the Invention
The present invention relates to a multi charged particle beam writing method and a multi charged particle beam writing apparatus, and for example, to a method of obtaining high accuracy of a plurality of irradiation positions of multiple beams.
2. Description of Related Art
The lithography technique that advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. The electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” a pattern on a wafer and the like with an electron beam.
As an example employing the electron beam writing technique, there is a writing apparatus using multiple beams (multi-beams). Compared with the case of writing a pattern by using a single electron beam, since a multi-beam writing apparatus can emit multiple radiation beams at a time, it is possible to greatly increase the throughput. In such a writing apparatus of a multi-beam system, for example, multiple beams are formed by letting an electron beam emitted from an electron gun assembly pass through a mask with a plurality of holes, blanking control is performed for each of the beams, and each unblocked beam is reduced by an optical system and deflected by a deflector so as to irradiate a desired position on a target object or “sample” (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2006-261342).
In such a writing apparatus of the multi-beam system, irradiation is performed with a plurality of beams at a time, and then, the irradiation positions of the multiple beams need to be highly precisely adjusted. For example, as to the size of a pattern to be written, it can be adjusted by altering the dimension of each beam by calibrating the reduction ratio of the lens in the optical system. However, if the lens condition is changed, a phenomenon such as a pattern rotation or a field distortion will occur. Therefore, it will be difficult to adjust the lens conditions to be in an optimized state together with a lot of other parameters necessary for the optical system to provide a highly accurate dimension. Then, if performing a rotation adjustment mechanically, it requires to accurately adjust a rotation position on the order of nm, which is not realistic. As to the field distortion, in the first place, there exists a theoretical field distortion in the optical system. For correcting this distortion, for example, it is necessary to perform a significantly accurate design in order to adjust a manufacturing accuracy of nm or below (e.g., 0.1 nm), which is also unrealistic. Moreover, even if the design is carried out highly precisely in the optical system design, restriction exists in the setting range of other design parameters. Therefore, if it is attempted to correct field distortion, there is a possibility of preventing the optimization of other conditions (for example, resolution performance, focus depth, and the like). Further, if it is attempted to attain equalization of a magnetic field in order to reduce distortion, an enormously large lens barrel will be needed, for example. Moreover, a lot of complicated correction systems are necessary for reducing the distortion, which may become an excessive burden to the apparatus. Furthermore, after manufacturing writing apparatuses, although adjustment for an actual apparatus becomes needed, since parameters for writing processing are intricately related to each other, a parameter for correcting distortion is not an independent variable even if the distortion is attempted to be corrected. Consequently, it will be difficult to execute the optimization or will take a lot of time even if the optimization can be performed.
As described above, in the writing apparatus of the multi-beam system, since irradiation is performed with a plurality of beams at a time, it is required to highly accurately adjust the irradiation positions of such multiple beams. In the multi-beam system, a pattern of a desired shape is formed by connecting the beams, which have been formed by passing through the same forming hole or different forming holes, at predetermined shot intervals by a raster scan method, for example. Thus, if a beam is shifted (deviated) from a desired irradiation position due to distortion of the optical system and the like, it becomes difficult to write a pattern highly precisely.